Copyrights are enforceable even if you don't register them, but you can't demand/receive punitive damages. If you haven't registered your work with the Library of Congress, you can still sue and win, but all you can collect is real damages (i.e., the amount of income lost because of the piracy).
However it is so expensive to pursue a copyright infringement suit that few ever come to court. If you are really worried about someone stealing your plans, don't post them on the internet. There is no other way.
If you choose to register your work, you can do so easily and fairly cheaply online here http://www.copyright.gov/eco/
There's lots of good, correct info at http://www.copyright.gov so you don't have to rely on internet advice.
<Chas>